This invention relates to magnetic resonance (MR) imaging apparatus.
Such apparatus includes a magnet for producing a main magnetic field in an examination region in which magnetic resonant active nuclei such as protons (hydrogen atoms) precess about the main magnetic field at a so-called Larmor frequency. An excitation pulse of RF energy at the Larmor frequency applied via a transmit coil in or surrounding the examination region excites such nuclei to resonance, and the resulting RF signals emitted by them are received by means of an RF receive coil. In some circumstances the same coil may be used for transmit and receive.
Data representing the spatial distribution of the excited nuclei, which represents the spatial distribution of tissue, can be collected by spatial encoding of the main magnetic field during excitation and during reception. From such data a two-dimensional or three-dimensional image of the part of the patient under examination can be built up.
The receive coil is tuned to the Larmor frequency at which the main magnetic resonant active nuclei are excited, and typically the coil has a high Q (quality factor) given by:       Q    =                  ω        ⁢                  xe2x80x83                ⁢        L            r        ,
where xcfx89 is the angular frequency to which the coil is tuned, L is its inductance, and r its effective resistance.
When loaded by the body of the patient, which tends to be conductive because of its watery nature, the effective resistance is increased and Q is accordingly decreased, perhaps by as much as 80% of its unloaded value.
A typical Q for a receive coil would lie in the range of from 10 to 100, when loaded. Such a high Q is necessary in view of the minute nature of the MR signal to be received. However, such a sensitive coil is more responsive to external disturbing factors, which can result in inaccuracies or inconsistencies in the data set, which would show up as spurious features (known as artefacts) on the final image.
It is for this reason that MR imaging apparatus has to be so carefully shielded. A typical specification of a room housing an MR machine is 120 dB rejection of RF frequencies in the band of interest. This actually adds a considerable amount to the financial outlay required to install an MR machine.
Another consequence of the high Q of the RF receive coil is that inconsistencies in a data set can be caused. Movement of the body, including that due to respiration, affects the loading of the coil, as more or less water-based material moves into and out of the vicinity of the coil. In fact, the variation of the signal with such movement is a relatively complicated matter, since the signal received by the coil when the body moves towards it is reduced according to the square root of the (decreased) Q of the coil but is increased in linear fashion according to the filling factor. The filling factor depends on how close the region to be imaged is to the receive coil. For a coil close to the body (a close-coupled coil), however, small changes in position have a much greater impact on Q than on the filling factor. (The converse is true if the coil is some way away from the body).
The variation of loading applies whether the receive coil is a large coil surrounding the body (a whole body coil), a coil placed on the surface of a region of interest of the patient (a surface coil) or a small internal coil such as those used in or with endoscopes, including flexible versions such as gastroscopes or colonoscopes. Especially with the latter, coil motion is hard to prevent relative to the volume to be imaged.
The invention provides magnetic resonance imaging apparatus, comprising a first RF coil for receiving magnetic resonance (MR) signals to enable an image to be produced, and an additional RF coil for receiving signals which provide data about the signal received by the first RF coil.
The additional coil enables an indication to be provided if a variation of the signal received by the first RF coil is due to external disturbances or patient movement.
The additional RF coil may be arranged to detect noise transients and/or may be tuned to the frequency of a reference transmitter, to enable the signal output of the first RF coil to be corrected for changes in Q of the coil due to movement of the patient. The additional RF coil is preferably superimposed on the first RF coil. A second additional RF coil may be provided, which also may be superimposed on the first RF coil, to enable broadband noise spikes to be detected in conjunction with the first additional RF coil.